1. Field of the Invention
The present invention relates to a phosphor having a coating made of pigment particles (hereinafter referred to as "pigment coated phosphor") and to a high contrast color television cathode ray tube using the pigment coated phosphor. More particularly, the present invention relates to a red pigment coated red emitting phosphor and to a high contrast color television cathode ray tube using the red pigment coated red emitting phosphor. 2. Description of the Prior Art
Heretofore europium activated yttrium oxysulfide (hereinafter often referred to as "Y.sub.2 O.sub.2 S:Eu") phosphors have been employed as red emitting phosphor particles in cathode ray tubes for color television. The Y.sub.2 O.sub.2 S:Eu phosphors vary in emission color and luminance depending upon the amount of the europium used as an activator (thereinafter simply referred to as "Eu activator value") as will be discussed hereinbelow. However, upon use of the phosphor as a red emitting phosphor in a cathode ray tube for color television, it is required from the viewpoint of the various characteristics desirable of cathode ray tubes, such as color reproduction, white luminance, red emitting color point, red emission luminance, and the like, that the phosphor possesses color points for emission in or in close proximity to a color area surrounded by color points A (x=0.643, y=0.357), B (x=0.643, y=0.343), C (x=0.652, y=0.340) and D (x=0.652, y=0.348) in the Standard Chromaticity Diagram Sytem CIE as is shown in FIG. 8 below. Thus, there has been provided for practical use as a red emitting phosphor for color television cathode ray tubes a Y.sub.2 O.sub.2 S:Eu phosphor having its color point within the red area described above wherein the Eu activator value is in the range of 0.067 to 0.08 gram atom per 1 mol of Y.sub.2 O.sub.2 S.
As is well known in the art, adherence of pigment particles of the corresponding color to at least one of the respective surfaces of the blue phosphor particles, green phosphor particles and red phosphor particles employed in a cathode ray tube for color television markedly enhances the contrast of the image formed on the cathode ray tube, since a part of the visible region of the emitted spectra is cut due to the filter effect of these pigment particles adhering thereto, resulting in a clearer emission color and further, absorption of incident external light is enhanced by the pigment coloration of the luminescent screen so as to reduce the reflection of light therefrom (U.S. Pat. No. 3,886,394).
It is required for the pigment coated phosphor employed in a cathode ray tube for high contrast color television mentioned above that reflectance be low and emission luminance be sufficiently high. That is, for a given reflectance, the emission luminance is desirably as high as possible.